Whether they are Diesel cycle, Otto cycle, two stroke or three stroke engines, internal combustion engines comprise at least one piston ring. The piston ring seals the space between the cylinder sleeve and the piston head, insulating the combustion chamber from the other internal engine components. The piston ring is arranged radially in the base of the piston head, preventing the combustion gases from escaping from the combustion chamber in the direction of the casing, and preventing the engine oil from penetrating the combustion chamber. Usually, engines are provided with three rings placed in parallel in grooves arranged in the base of the head.
Certain internal combustion engines, principally those operating on a diesel cycle, work under high load. These engines operate at low speed with high power and therefore are more demanding in terms of their mechanical components. Thus, the rings used in such high-power engines need to be highly wear-resistant, not only on the external surface but also on the upper and lower surfaces of the rings.
Prior art piston rings generally comprise a stainless steel base with a nitride coating deposited on the external, internal, upper and lower surfaces and on the chamfers of the rings.
Patent document DE 10 2005 023 627 discloses a piston ring provided with a nitride coating and a hard chromium coating deposited on the external surface of the ring. However, the document does not disclose a complete solution since the ring, which is the subject matter of the invention, has only one of the edges of the external surface chamfered and with a hard chromium coating. That is to say, the hard chromium coating is applied partially on the external surface, thereby reducing wear-resistance.
Furthermore, documents JP 3090520 and U.S. Pat. No. 6,508,473 disclose piston rings provided with a nitride coating applied to the internal, upper, and lower surfaces, and a hard chromium coating applied using a physical vapor deposition (PVD) process to the external surfaces of the rings.
These documents disclose, as disadvantage, PVD coatings that are limited to smaller thicknesses and thus coatings that are restricted in use in terms of durability. In addition to promoting the ring fatigue process through the compressive action of the PVD layer on the nitriding.
It is therefore necessary to find a piston ring comprising a nitride coating combined with a plated coating composed of chromium deposited in the most critical region of the ring, which is defined as the external surface and the chamfered edges, the ring offering a high level of durability owing to excellent resistance to wear on the various faces and the respective, sleeve and piston, contact moments.